Corresponding author: Dr M.F. Kinnaird. Email: wcs-ip@indo.net.id

Helmeted Hornbills Rhinoplax vigil are unique among the Bucerotiformes in having solid "ivory" casques that are used sometimes in aerial displays where individuals collide casque-to-casque in mid-air (Kemp 1995). Knocking and abrasion during these aerial combats may even help to shape the distinctive flat, front profile of the adult birds' casques (Cranbrook & Kemp 1995). The behaviour has been reported only among males, although females have bills and casques of similar structure and proportions to males (Sanft 1960, Kemp 1995). Interactions, which may last for up to 2 hrs, have been attributed to agonistic interactions and territorial defense (Kemp 1995) and to intoxication from consumption of fermented figs (Schneider 1945). Published descriptions of aerial casque-butting by Helmeted Hornbills are rare, and most observations are recounted tales of other observers or, in the case of Kemp (1995) dramatized stories to illustrate the diverse behaviour of the hornbill family (Cranbrook & Kemp 1995). In this note, we describe six acts of aerial casque-butting, hereafter referred to as aerial jousting, by Helmeted Hornbills. For all jousts, we recorded the time of day, whether the jousting took place in or near fruiting trees, and the number and sex of the 'actors'. We also report on the time and location of five additional jousting events that were investigated only after we were made aware of the behaviour by the distinctive clacking sounds of colliding casques.

The majority of observations were made over a 2-month period (Sept 98 - Oct 98), as part of a larger study on hornbill foraging ecology (Hadiprakarsa 2000) at the Way Canguk Research Site, Bukit Barisan Selatan National Park (BBS), Sumatra, Indonesia. Three additional records were made at Way Canguk in 2000 and 2001, and one was noted at the Budo-Sungai Padi National Park (BSP), Thailand in 2000. The Way Canguk Research Site covers approximately 900 ha of disturbed and undisturbed lowland rainforest habitat in the southeast portion of the 3,568 km2 BBS (Kinnaird & O'Brien 1998). BSP is a small forest isolate on a steep mountain outcrop (341 km2) near the border between Thailand and Malaysia and is similar to BBS in being a mix of disturbed and undisturbed rainforest habitat (Uthai Treesucon, Thailand Hornbill Project, unpubl. data).

Visual and auditory records were spread throughout the day, but the majority occurred in the morning (Table 1). Jousting events generally involved one male-male pair. Before jousting, perched males were observed hitting tree branches with their bills and rubbing their bills from side-to-side on the branches as though they were drilling for insect larvae or bill cleaning. In two instances, males left their perches immediately after hitting tree branches, flew in opposite directions, circled, glided towards one another, and collided casque-to-casque in mid air, then returned to their previous perch before repeating the aerial jousting. In BSP, Thailand, however, males hit tree branches "like a boxer punching his gloves at the corner of the ring before a fight begins", then called for approximately 30 minutes before ascending into the air, jousting, and returning to the same perch. When collisions occur, the resulting sound (a loud "CLACK!") can be heard in the forest understory at least 100 m away. The collision, which generally occurs during a glide and not while flapping the wings, can be so powerful that one or both birds are thrown backwards, performing dramatic, acrobatic flips before righting themselves and flying level (Fig. 1). Combatants were recorded approaching one another at distances of 25-50 m.

On three occasions we observed more than one male-male pair jousting simultaneously but we could not determine whether partners switched among the pairs or jousted continuously with the same individuals. Jousting among more than one pair of combatants took longer (up to 50 minutes) and usually involved a greater number of actual jousting events (2-12 collisions) than single pair interactions (1-4 collisions).

During at least two jousting events, females perched near the combating males and were observed flying 1-2 meters above or to the side of their males as they began jousting (Fig. 2a). When the males collided, the females veered off in opposite directions and returned to their previous perches to be joined immediately by their males (Fig. 2b). On one occasion, we observed jousting between a male and female Helmeted Hornbill. The female was perched in the upper canopy of a fruiting fig preening while the male circled in a large loop overhead, then flew straight towards the female and collided with her casque-to-casque. Immediately following, the male perched next to the female and both preened their feathers.

Three of the six direct observations of jousting events occurred in or near fruiting, hemiepiphytic fig trees (Ficus spp.). Of the remaining three, one occurred near an active nest site between the breeding male and a male intruder and the other two occurred near a large, leafless tree snag. Three of the five auditory records also occurred near large fig trees with ripe fruit crops. Therefore, seven of the eleven documented jousting events were associated with defendable resources.

Although our data are not conclusive, we believe they indicate that aerial jousting may have developed as an agonistic behaviour during resource competition and not as a territorial display per se. Helmeted Hornbills may compete for nesting sites and food resources, particularly fruiting figs. Because Helmeted Hornbills prefer to nest in large trees (diameter breast height: 105-216.6 cm) of the family Dipterocarpaceae, and choose only those trees with distinctive knobs or stumps at the nest entrance (Thiensongrusamee et al. 2001), nest sites are limited and competition is likely. Helmeted Hornbills have been described as fig specialists (Kemp 1995) and studies in peninsular Malaysia and on Borneo and Sumatra confirm the fruit portion of the diet to be 98% to 100% Ficus, with the animal portion less than 1% in some areas (Hadiprakarsa 2000, Lambert 1989, Leighton 1982). Such heavy reliance on widely dispersed, low-density resources with unpredictable, asynchronous fruiting patterns (Jansen 1979) would seem to preclude year-round territoriality. Disappearance of Helmeted Hornbills from the Way Canguk Research Site during certain months, and congregations of more than eight pairs together in one tree, reinforce the idea that these birds are not strictly territorial (Anggraini et al. 2000, O'Brien pers.com, Hadiprakarsa unpubl. data). The "territorial calls" (Kemp 1995) of Helmeted Hornbills may function to coordinate mated pairs that often forage far apart (Leighton 1982) or to announce a pair's position to competitors whose home ranges may overlap, much like the loud-calls of non-territorial Cercocebus primates (Waser 1976). Alternatively, Anggraini et al. (2000) speculate that Helmeted Hornbills may practice facultative territoriality, or variable resource defense, exhibiting territoriality only when resources are limiting and defendable.

Our observations of male-female and male-male jousting in and near fruiting figs support the idea that the behaviour may best be explained in the context of competition for food resources. Simultaneous jousting among several males also does not rule out resource defense but argues strongly against territorial behaviour unless group defense is occurring. More data are necessary to evaluate the function of aerial jousting among Helmeted Hornbills; we hope that our ideas stimulate researchers to examine more thoroughly the behavioural correlates of this little-known Asian hornbill.

ACKNOWLEDGEMENTS
We are grateful to Alan Kemp and Pilai Poonswad for encouraging us to publish these observations, to Tim O'Brien, Alan Kemp and George Schaller for helpful comments, and to Mohammad Iqbal, Risdianto and Wariyono for assistance in the field. We thank our partners in Indonesia, the Directorate for Forest Preservation and Nature Conservation, for permission to work in Bukit Barisan Selatan National Park. The Wildlife Conservation Society (MK &YK) and the Thailand Hornbill Project (PT) have kindly sponsored and funded our hornbill studies over the years.